A failure detection device is used to detect a failure of an inverter. For example, JP-2006-81327-A (corresponding to US 2006/0056206) describes an inverter failure detection device that determines a failure of an inverter on the basis of a sum of terminal voltages of respective phases while the inverter is driven under PWM (Pulse Width Modulation) control.
Above-mentioned JP-2006-81327-A, however, neither describes nor implies in which manner a failure detection is performed when a motor is not driven. Further, in the case of a motor applied to, for example, an electric power steering device, the motor may possibly rotate even before the motor is driven in response to an operation on a steering wheel by a driver. When a rotating electric machine is set in motion by an external force, the inverter failure detection device may possibly fail to perform a pre-drive failure detection normally due to the influence of a counter-electromotive force.